A speciall designed fiberoptic probe that is capable of monitoring intramyocardial pH is of unique importance to the study of myocardial ischemia. A partial coronary artery occlusion was produced in open chest anesthetized dogs and myocardial blood flow was determined by the use of trace microispheres. Heart rate, mean aortic pressure, coronary blood flow, coronary perfusion pressure, cardiac output, and peak ejection rate were monitored. Gated blood pool scanning was performed with a gamma camera to examine global ejection fraction, regional wall motion, and regional ejection fraction. The calcium antagonist drug, verapamil, was administered in progressively increasing doses to determine its effects on lischemia. Blood levels of verapamil were drawn and quantitated. Verapamil was found to increase the depressed intramyocardial pH in the ischemic area presumably reflecting a beneficial effect of the drug on the severity of the ischemic insult.